WO1998005599A1

WO1998005599A1 - Process and apparatus for modifying and homogenizing glass melts

Info

Publication number: WO1998005599A1
Application number: PCT/GB1997/002034
Authority: WO
Inventors: David Martlew; Robert Emmett Trevelyan; Peter James Whitfield; Stanley Lythgoe; John Caines
Original assignee: Pilkington Plc
Priority date: 1996-08-03
Filing date: 1997-07-25
Publication date: 1998-02-12
Also published as: AU3702197A; JPH11513972A; BR9706541A; ID17975A; EP0853597A1; CN1200713A; KR19990063970A; ZA976709B

Abstract

A method of modifying a base glass composition so as to change its properties in which molten glass is caused to flow in a stream unidirectionally through a substantially horizontal channel with the modifying material then being added thereto. The horizontal and vertical distribution of the modifying material in the glass stream are effected separately. The molten glass with the modifying material homogeneously distributed therein is then delivered to a forming facility.

Description

PROCESS AND APPARATUS FOR MODIFYING AND HOMOGENIZING GLASS MELTS

This invention relates to the manufacture of glass and more particularly to modifying a base glass to change its properties, for example to impart a desired colour.

The concept of adding a colourant to a molten base glass is already known but is believed to have been applied in practice in the field of container glasses (where it is sometimes referred to as 'forehearth colouring') rather than in the flat glass field. This is largely because the quality and especially homogeneity and bubble requirements for flat glass are generally higher than those for container glass and for float glass are continually becoming more stringent. In particular, apart from special 'artistic' type products, coloured flat glass and especially float glass needs to have a uniform tint throughout its area and added colourant material therefore needs to be very uniformly distributed within the glass. Further, the addition and distribution needs to be effected in a manner which does not result in unacceptable bubble. Nevertheless, there have been proposals for adding colourant material to a molten base glass composition to produce coloured flat glass. It will be understood that, if this can be done successfully, it can greatly reduce the changeover time from making clear flat glass to making coloured flat glass, or for effecting a tint change from one colour to another, which is required when a different composition has to be melted in the basic melting tank or furnace.

EP 0 599403A discloses an arrangement for making coloured glass containers, EP 0 556 576A discloses a colouring arrangement for window glass or vessel glass compositions and EP 0 275 534A discloses an arrangement for making coloured flat glass. All these arrangements have some form of action to mix the colourant additive material in the molten glass. EP 0 599 403A has pulsed bubblers while EP 0 556 576A, which is particularly concerned with avoiding bubbles, has an array of like mechanical stirrers. EP 0 275 534A also has an array of like mechanical stirrers. It will be understood that each of the stirrers in such an array imparts the same type of stirring action to the glass. Such one type of stirring action may not achieve the required uniformity of distribution of the colourant material within the glass.

According to the present invention there is provided a method of modifying a base glass to change its properties comprising flowing molten base glass substantially unidirectionally in a stream along a substantially horizontal channel, adding modifying material to the horizontally flowing molten base glass and distributing the modifying material vertically in the molten glass, separately distributing the modifying material horizontally in the molten glass with a component transverse to the direction of travel of the stream, and delivering the stream of molten glass with the modifying material distributed substantially homogeneously therein to a forming facility. By separately causing vertical and horizontal distribution of the modifying material in the molten glass, a more even dispersion may be achieved and in a manner which avoids unacceptable bubble.

The modifying material may be added to the molten base glass and then separately distributed vertically in the molten glass. This vertical distribution may be effected by stirring the horizontally flowing molten base glass carrying the modifying material so as to cause relative vertical movement within the glass.

The vertical stirring may be performed by rotating substantially vertical shafts carrying substantially helical blades at least partially immersed in the molten glass. Such mechanical stirrers may be made of a refractory material, and preferably a refractory metal such as platinum.

The modifying material may be distributed horizontally in the molten glass by stirring the horizontally flowing molten base glass carrying the modifying material so as to cause relative horizontal movement within the molten glass with a component transverse to the direction of travel of the stream.

The horizontal stirring is preferably performed by rotating substantially vertical shafts carrying paddle blades at least partially immersed in the molten glass.

Preferably stirring to cause relative vertical movement is carried out prior to separate stirring to cause relative horizontal movement. In other words the vertical stirring is effected upstream of the horizontal stirring with respect to the flow of the stream of molten glass.

The base glass may be substantially clear glass or a tinted glass and the modifying material may be a colourant material so as to produce a tinted glass or modify the tint of a tinted base glass. It will be appreciated that uniform distribution of the colourant material will produce a uniform tint.

The modifying material is preferably in molten form as it is added to the molten base glass. It may be slid onto the upper surface of the substantially horizontally flowing stream of molten base glass. Alternatively, it may be added beneath the surface of the molten base glass, e.g. by introduction from a member whose feed end is immersed in the molten glass, and may be added in a manner which distributes it vertically in the molten glass as it is added.

The forming facility may be a flat glass (and especially a float glass) forming facility and the method may comprise forming the molten glass with the modifying material distributed substantially homogeneously therein into flat glass (and especially float glass). The invention further provides flat glass (and especially float glass) produced by the process.

Another aspect of the invention provides apparatus (suitable for use in the above method according to the invention but not exclusively) for adding modifying material to a substanti,ally horizontally flowing stream of molten base glass comprising a feed member having a lower portion for immersion in the molten glass and with an outlet for modifying material, and means for feeding modifying material into the feed member to emerge through the outlet into the molten glass stream beneath its surface, in which the outlet extends with a vertical component so as to distribute the modifying material over at least a major part of the depth of the stream.

The outlet may be on one side of the feed member which in use faces in the direction of travel of the stream and preferably comprises a series of holes which may, for example, be disposed in a substantially vertical line. The modifying material preferably emerges from the outlet in a molten state and the apparatus may comprise a melter to melt the modifying material and deliver it in a molten state to the feed member. The melter preferably includes a filter device to prevent unwanted matter being delivered to the feed member. The feed member is conveniently a pipe.

Embodiments in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic plan view of a glass melting furnace or tank Figure 2 is a schematic vertical section through a feeder for modifying material Figure 3 is a schematic vertical section through an alternative form of feeder for modifying material Figure 4 is a schematic vertical section through another form of feeder for modifying material Figure 5 is a schematic plan view of a glass melting furnace or tank, and Figure 6 is a schematic plan view of the entrance to a float glass forming facility or float bath The glass melting furnace or tank schematically shown in Figure 1 comprises an upstream part 1 having a melting zone 2 and a refining zone 3 connected by a waist 4 to a working end 5. A canal 6 leads from the working end 5 to an exit 7 at the entrance to a forming facility. Batch material is fed into the melting zone 2 in well known manner and is melted there to form molten glass which is then refined, i.e. bubbles are removed, in the refining zone 3. The molten glass passes through the waist 4 to the working end 5 and then travels along the canal 6 to the exit 7, conditioning of the glass taking place in the working end 5 and the canal 6. Those skilled in the art will understand that there are generally not precise fixed boundaries between the melting, refining and conditioning zones.

The waist 4, working end 5 and canal 6 form a substantially horizontal channel through which molten base glass melted and refined in the tank part 1 flows in a stream substantially unidirectionally, i.e. downstream, towards the exit 7 without any substantial return upstream flow. Preferably the molten glass stream also flows unidirectionally in the downstream part of the refining zone 3, for example over a distance upstream of the waist 4 about the same as the stream width. Feeders 8 located in the waist 4 add modifying material, e.g. colourant, to the horizontally flowing molten base glass and the modifying material is distributed vertically in the molten glass by helical stirrers 9 located towards the upstream end of the working end 5. The stirrers 9 comprise vertical shafts carrying helical blades at least partially immersed in the molten glass so that rotation of the shafts effects stirring of the horizontally flowing molten base glass carrying the modifying material so as to cause relative vertical movement within the molten glass which distributes the modifying material vertically.

Downstream of the stirrers 9 and towards the upstream end of the canal 6 are further stirrers 10 which are designed to distribute the modifying material horizontally in the molten glass with a component transverse to the direction of travel of the stream, i.e. laterally or transversely. This horizontal distribution is carried out separately from the vertical distribution effected by the helical stirrers 9. The stirrers 10 comprise vertical shafts carrying substantially vertical paddle blades at least partially immersed in the molten glass so that rotation of the shafts effects stirring of the horizontally flowing molten base glass carrying the modifying material so as to cause relative horizontal movement within the molten glass with a component transverse to the direction of travel of the stream which distributes the modifying material horizontally across the width of the stream.

The helical stirrers 9 and the paddle stirrers 10 are each of a form known per se and are made of suitable refractory material and preferably of refractory metal such as platinum. The helical stirrers 9 are shown as a pair rotated in the same direction (indicated by the arrows as anticlockwise) while the paddle stirrers 10 are shown as a pair rotated in opposite directions (indicated by the arrows) in a manner which tends to drive the molten glass between them. It will be appreciated, however, that the number of stirrers (across and along the stream) and their direction of rotation may be selected to suit particular requirements but should generally be sufficient effectively to cover the full width of the molten glass stream at the stirring locations. It will further be appreciated that in practice purely horizontal or purely vertical relative movement within molten glass may be difficult, if not impossible, to achieve by the respective different stirrers and references to horizontal and vertical movement herein are intended to indicate predominantly horizontal and predominantly vertical movement respectively. It will further be appreciated by those skilled in the art that by effecting (predominantly) vertical motion and (predominantly) horizontal motion separately by the respective different stirring actions produced by the stirrers 9 and the stirrers 10, a relatively simple overall arrangement can be employed which compares favourably with the more complicated numerous arrays of like complex stirrers as are sometimes used in the manufacture of glass containerware.

In the Figure 1 embodiment, the temperature of the glass at the location of the stirrers 9 may typically be in the range 1200°C to 1450°C, for example about 1300°C. At the location of the stirrers 10 the molten glass temperature may be in the range 1150°C to 1400°C, for example about 1280°C. The longitudinal distance between the location of the stirrers 9 and the location of the stirrers 10 is sufficient to avoid impairment of mixing and, from this point of view, is preferably at least the width of the molten glass stream, which may typically be in the range lm to 4m. However, it may be desirable to make this distance longer to avoid substantial adverse interaction between the stirring operations which might, in particular, cause bubble. Thus the distance is preferably greater than twice the width of the molten glass stream, i.e. of the working end 5, and typically may be in the range 2m to 8m, for example about 4m. The depth of the molten glass stream is such as to be consistent with unidirectional flow and may typically be in the range 200mm to 800mm, for example about 500mm with a long canal (e.g. of 50m length) or about 250mm with a shorter canal (e.g. of 10m length), at the stirring locations.

The location of the stirrers 9 is not particularly critical but may, for example, be a distance downstream from the feeders 8 about equal to the width of the molten glass stream, i.e. of the working end 5.

Figure 1 schematically shows three feeders 8 spaced across the width of the waist 4 where the molten glass temperature may typically be in the range 1200°C to 1480°C, for example about 1320°C.

It will be understood that any suitable number of feeders may be provided and that the feeders may take any suitable form. Preferably they are such that the additive material is in molten form as it is added to the molten base glass. One simple such form of feeder is schematically shown in Figure 2. It comprises a vertical tube 11 having a funnel 12 at the top. The tube is carried in a refractory roof 13 over the waist 4 (whose base is shown in Figure 2) and the bottom of the tube 11 dips into the molten glass so that its end 14 is beneath the glass surface S. There is an atmosphere space between the roof 13 and the molten glass surface S through which the tube 11 passes and this provides a sufficiently hot environment to melt the additive material in the tube. In use additive material of suitable form. e.g. tablets, is fed into the funnel 12 in any suitable convenient manner and melts as it travels by gravity down the tube 11. It emerges from the bottom end 14 of the tube (as indicated by the row of arrow heads) beneath the surface S, and therefore into the body of the stream of molten base glass flowing (as indicated by the arrow) towards the stirrers 9 (Figure 1).

Figure 3 schematically shows an alternative form of feeder which is a modified version of that shown in Figure 2 and has the same reference numerals indicating like parts. The Figure 3 version differs from that of Figure 2 at the bottom of the tube 11. In Figure 3 the tube ends in a conical portion 15 with a central hole leading into a vertical pipe 16 having an associated guide 17 which is shaped smoothly to guide the molten additive material from a vertical path into a horizontal path. The bottom end 18 of the pipe 16 is spaced a short distance above the level of the molten glass surface S so as to give a drop height preferably less than three times the orifice diameter of the pipe, which orifice diameter may, for example, be of the order of 10mm. The bottom end 19 of the guide 17 is at the level of the molten glass surface S. The additive material melted as it travels by gravity down the tube 11 passes through the central hole in the conical portion 15 into the pipe 16 to emerge from its bottom end 18 from which it falls onto the guide 17. The molten additive material is then gently slid onto the surface S of the molten base glass from the end 19 of the guide 17.

It will be appreciated that by causing the molten additive to be introduced beneath the glass surface as in Figure 2 or slid onto the glass surface as in Figure 3 bubble problems which might otherwise occur (e.g. if the additive material were allowed to fall directly onto the glass) can be avoided or minimised. Figure 4 schematically shows a further form of feeder which introduces the additive material beneath the molten base glass surface. It comprises a vertical feed member in the form of a pipe 20 having a lower end portion 21 dipping beneath the glass surface. The extreme bottom end 22 of the pipe is closed and the immersed end portion 21 has on one side an outlet which extends in a vertical direction formed by a series of small holes 23 arranged in a vertical line and facing in the direction of travel of the molten glass stream (as indicated by the arrow). The additive material in a molten state in the pipe 20 emerges through the holes 23 (which may, for example, be about 1 mm in diameter) into the molten base glass. Since the holes are distributed vertically along the immersed end portion 21 of the pipe, the additive material is distributed vertically in the molten glass over a major part and preferably substantially the full depth of the stream as it is added.

The top of the pipe 20 carries an increased diameter head portion 24 above which is a hollow melter device having an inclined part 25 and a horizontal part 26 located in a hot environment. If desired the hollow melter device may be directly heated by electricity, although alternative methods of heating could be employed. A pair of overlapping baffles 27 and 28 are arranged in the horizontal part 26 just upstream of an exit pipe 29 in its bottom which feeds into the head portion 24 of the pipe 20. The inclined part 25 of the melter device has an entry 30 into which, in use, tablets of additive material are fed for melting. The molten additive material passes along the horizontal part 26 to the baffles 27 and 28 which it can pass only by flowing beneath the upper baffle 27 and then over the lower baffle 28. By such arrangement, with the lower edge of the upper baffle 27 at a lower level than the upper edge of the lower baffle 28, the baffles form a filter device against scum and other unwanted matter permitting only clean molten additive material to pass between them. Such material then flows through the exit pipe 29 into the head portion 24 of the pipe 20 and down that pipe to emerge through the holes 23 in its lower portion 21 as already described.

Since with this embodiment of feeder the molten additive material is distributed vertically in the molten base glass as it is added to it, it may be unnecessary to provide additional vertical distribution, for example by means of helical stirrers as previously described. Thus, Figure 4 schematically shows one of a pair of paddle stirrers having a vertical shaft 31 carrying projecting vertical paddles 32 at its lower end. The shafts 31 are rotated (as indicated by the arrow) and the immersed paddles 32 stir the horizontal flowing molten base glass carrying the modifying material so as to cause relative horizontal movement within the molten glass with a component transverse to the direction of travel of the stream. This distributes the modifying material horizontally in the molten glass.

The feeder pipe 20 and the stirrer shaft 31 are suitably mounted to pass through a roof 33 over the substantially horizontal channel along which the molten glass stream flows and whose base is indicated as 34 in Figure 4. The upper portion of the pipe 20, where it passes through the refractory roof structure 33, is directly heated by electricity in order to ensure that a temperature is maintained at which the liquid additive stream will continue to flow. Figure 4 further shows electrical heaters 35 mounted in the atmosphere space between the roof 33 and the molten glass surface S which also heat the pipe 20 to ensure flow of the liquid additive.

A number of pipes 20 and a number of paddle stirrers may be spaced across and/or along the glass stream. The paddle stirrers are preferably located a relatively short distance downstream from the pipes 20 so that they draw the molten glass carrying the modifying material towards them and prevent any substantial vertical displacement, e.g. sinking, of the modifying material.

It will be appreciated that the described vertical line of holes 23 in the lower end portion 21 of the pipe 20 is given by way of example and other outlet arrangements could be employed. The outlet need not necessarily face in the direction of travel of the stream and there could, as another example, be a series of holes disposed round the circumference of the pipe which could, if desired, be rotated. Further, the pipe need not necessarily be vertical but could be inclined in whole or in part so that the outlet still extends with a vertical component In particular, the immersed end portion 21 could be angled along the direction of travel of the stream so that its bottom end 22 is further downstream than its upper end. The unimmersed portion of the pipe 20 could similarly be angled if a straight pipe is used or could be vertical with a bend in the region of the glass surface S to provide the inclined lower portion 21. Various other arrangements to suit particular required geometries are possible. Also, the modifying material need not necessarily be melted separately as described with reference to Figure 4 but could be melted in a tube 11 as described with reference to Figure 3. A shorter pipe 20 of the Figure 4 form could be connected to the bottom end of the tube 11 in Figure 3 in place of the pipe 16 and guide 17. If desired an arrangement could be attached to apply vacuum or reduced pressure to the tube 11 so as to have a de-gassing effect on the material contained in it so as further to reduce the risk of bubble.

Figure 5 is a schematic view similar to Figure 1 and uses the same reference numerals to indicate like parts. The Figure 5 arrangement has a feeder and paddle stirrers as described with reference to Figure 4, both located in the working end 5. For ease of illustration Figure 5 shows a single feeder 36 and a single pair of paddle stirrers 37 whereas, in practice, a plurality of feeders, for example two or three, could be employed and there would normally be a respective pair of paddle stirrers associated with each feeder. With a plurality of feed pipes across the width of the molten glass stream, they are preferably spaced from each other by a distance equal to or less than the spacing of the paddle stirrer shafts so that each feeder is clearly associated with a particular pair of paddle stirrers.

The longitudinal distance of the stirrers 37 from the feeders 36 is preferably less than the spacing between the stirrer centres which may typically be in the range 300mm to 1.3m, for example about 600mm. The temperature of the molten glass at the location of the stirrers 37 may typically be in the range 1100°C to 1400°C, for example about 1180°C.

Additional paddle stirrers corresponding to the stirrers 10 in Figure 1 may be retained in the Figure 5 embodiment towards the upstream end of the canal 6. If desired further paddle stirrers 38 may be provided towards the downstream end of the canal 6 in both the Figure 1 and the Figure 5 embodiments to effect further horizontal distribution of the modifying additive material.

Figures 1 and 5 generally indicate an exit 7. The stream of molten glass with the modifying material distributed substantially homogeneously therein is delivered from the exit to a forming facility. Figure 6 schematically indicates a float glass forming facility comprising a float bath 39 to which molten glass is delivered from a spout 40 with an associated control tweel 41. For such a facility the exit 7 in Figures 1 and 5 is connected with the spout 40 of Figure 6 and the delivered molten glass with the modifying material distributed substantially homogeneously therein is formed into float glass in well known manner. Bubble problems are avoided or minimised by first refining the molten glass and then adding and distributing the modifying material in a manner as described. The numerical values of temperature and dimensions given by way of example above all relate to a glass melting furnace or tank for a float glass forming facility.

Although, having regard to the high quality requirements for float glass, the invention is particularly useful for float glass, it could be applied to other forms of flat glass such as rolled plate or drawn sheet. The invention could also be applied to other types of glass products for example containerware or television tubes. In each case the exit from the glass melting furnace or tank is connected with an appropriate forming facility.

The modifying material may, as previously indicated, be a colourant material. The base glass to which the colourant is added can be a clear glass so that the colourant imparts a tint or the base glass may itself already be tinted so that the colourant modifies the tint In the latter case, cullet of the modified tint may be recyclable into the melting tank for the base tint However, modifiers of properties other than colour (e.g. refractive index) could be used.

It will be understood that with flat glass which is normally viewed through the thickness of the glass a substantial change in tint (or other property) over the area of the glass is readily discernible whereas a change through the thickness is generally of less concern. Even distribution of the modifying material over the area of the glass is, therefore, more important than even distribution through its thickness and references herein to substanti∑dly homogeneous distribution in relation to flat glass are to be construed accordingly.

It will be appreciated that the accompanying drawings, especially Figures 1 and 5, are purely schematic and not to scale. The method of the invention can be used with a wide variety of glass melting tanks or furnaces and Figures 1 and 5 diagrammatically show one for the purposes of illustration. It will therefore be understood that the substantially horizontal channel (provided by elements 4, 5 and 6 in Figures 1 and 5) may, but need not necessarily, have changes in width and/or depth along its length, such changes preferably normally being gradual rather than stepped. Further, the unidirectional flow of the molten glass stream preferably commences upstream of the modifying material addition (e.g. in the refining zone 3 in Figures 1 and 5).

Variations from the embodiments specifically described which can be made without departing from the principles of the invention will be apparent to those skilled in the art. For example, although it will usually be preferable to effect vertical distribution of the modifying material in the molten glass before effecting horizontal distribution, there may be circumstances where horizontal distribution can usefully be effected before vertical distribution. Also, distribution of the modifying material in the molten glass could be effected by means other than those specifically described. Thus, in particular, vertical distribution could be caused by convection currents produced by heaters, such as electrodes, rather than by mechanical stirrers or there could be a combination of electrodes and stirrers. Further, while in the described embodiments the glass melting furnace or tank is indicated as feeding a single unit, one tank could feed a plurality of outlets connected with a plurality of forming facilities. If desired the glass modification could take place in a channel between the melting furnace or tank and the respective forming facility so that different facilities or lines could produce different products, for example as described in United Kingdom Patent Application No. 9616364.7.

Claims

1. A method of modifying a base glass to change its properties comprising causing a molten base glass to flow substantially unidirectionally in a stream along a substantially horizontal channel and adding modifying material to the horizontally flowing molten base glass characterised by distributing the modifying material vertically in the molten glass, separately distributing the modifying material horizontally in the molten glass with a distribution component transverse to the direction of travel of the stream, and delivering the stream of molten glass with the modifying material distributed substantially homogeneously therein to a forming facility.

2. A method according to Claim 1 characterised in that the modifying material is added to the molten base glass and is then separately distributed vertically in the molten glass.

3. A method according to Claim 2 characterised in that the modifying material is distributed vertically in the molten glass by stirring the horizontally flowing molten base glass carrying the modifying material so as to cause relative vertical movement within the molten glass.

4. A method according to any preceding claim characterised in that the modifying material is distributed horizontally in the molten glass by stirring the horizontally flowing molten base glass carrying the modifying material so as to cause relative horizontal movement within the molten glass with a component transverse to the direction of travel of the stream.

5. A method according to Claim 3 and Claim 4 characterised in that stirring to cause relative vertical movement is carried out prior to separate stirring to cause relative horizontal movement

6. A method according to any preceding claim characterised in that the additive material is in molten form as it is added to the molten base glass.

7. A method according to Claim 6 characterised in that the molten additive material is slid onto the surface of the molten base glass.

8. A method according to Claim 6 characterised in that the molten additive material is added beneath the surface of the molten base glass.

9. A method according to Claim 8 characterised in that the additive material is added in a manner which distributes it vertically in the molten glass as it is added.

10. A method according to any preceding claim characterised in that the forming facility is a flat glass, optionally a float glass, forming facility and comprising forming the molten glass with the modifying material distributed substantially homogeneously therein into flat glass, optionally float glass.

11. Rat glass, optionally float glass, produced by a process according to Claim 10.

12. Apparatus for adding modifying material to a substantially horizontally flowing stream of molten base glass (4,5,6) characterised by a feed member (8,1 1,20) having a lower portion (14,21) for immersion in the molten glass and with an outlet (18,19) for modifying material, and means for feeding modifying material into the feed member to emerge through the outlet into the molten glass stream beneath its surface(s), in which the outlet extends with a vertical component so as to distribute the modifying material over at least a major part of the depth of the stream.

13. Apparatus according to Claim 12 characterised in that the outlet (18,19) is on one side of the feed member (20,21,22) which in use faces in the direction of travel of the stream(s).

14. Apparatus according to Claim 12 or Claim 13 characterised in that the outlet comprises a series of holes (23).

15. Apparatus according to any of Claims 12 to 14 characterised by a melter (25,26) to melt the modifying material and deliver it in a molten state to the feed member.

16. Apparatus according to Claim 15 characterised in that the melter includes a filter device (28) to prevent unwanted matter being delivered to the feed member.

17. Apparatus according to any of Claims 12 to 16 characterised in that the feed member is a pipe (11,16,20).